The present invention relates to adhesive compositions for thermoplastic substrates and a method of manufacture thereof. More particularly, the present invention relates to such adhesive compositions comprising poly(arylene ether) polymers.
There remains a continuing need in the art for thermosetting adhesives for use with thermoplastic adherends, particularly substrates used in, and associated with, fabricating printed circuit boards. The need for miniaturization of circuit boards has increased and methods such as incorporation of microvias in printed circuit boards have been developed. Furthermore, it would be beneficial to have substrates capable of microvia technology and capable of being laser drilled without interference of reinforcements such as woven or chopped fiberglass.
In the fabrication of printed circuit boards, it is known to use copper foil coated with adhesive, or copper foil with a cured adhesive which is then coated with adhesive and partially cured as described in U.S. Pat. No. 5,557,843. This method is limited by the thickness of the copper foil that can be effectively coated and handled for practical use in the printed circuit board or rigid-flex board. Thermoplastic films are known which are metallized with copper or other conductive material to low thicknesses such as 3 microns or 5 microns. Examples of metallized polyimides are described in, for example, U.S. Pat. Nos. 4,842,946 and 4,959,121. These metallized films can be coated with adhesive and used, for example, in microvia and rigid-flex applications.
Adhesives suitable for bonding thermoplastic adherends, and circuit boards in particular, need to have good solvent resistance, toughness, and adhesiveness. The partially cured substrate used to manufacture circuit boards must have the characteristics of minimal flow under heat and pressure to provide adequate flow and fill, resistance to cracking, and the ability to develop adequate bond strength to all adherends during cure. When used with circuit boards, adherends include thermoplastic film, copper and/or treated copper for bond improvements, and other resinous plastic substrates used in the circuit board. When used in printed circuit boards, the dielectric loss and expansion properties of the adhesives should be adjustable to closely match those of the substrate. U.S. Pat. No. 3,900,662 to Yuan discloses an acrylic based adhesive, but such adhesives have relatively low glass transition temperatures and high expansion values at soldering temperatures. Thermosetting poly(arylene ether) resin compositions have a number of the desired properties, but lower molecular weight poly(arylene ether)s useful for impregnating reinforcement materials are brittle in partially cured form while higher molecular weight poly(arylene ether)s have difficulty forming films at room temperature due to high viscosity and tend to be brittle in partially cured form. U.S. Pat. No. 4,853,423 to Walles, for example, discloses poly(arylene ether) resins and polyepoxy resins together with novolac resins for circuit boards. More recent patents directed to poly(arylene ether) resins, for example U.S. Pat. No. 5,834,565 to Tracy et al., disclose use of poly(arylene ether) polymers in circuit board substrate materials having number average molecular weights of less than about 3,000. Commercially available poly(arylene ether) resins have number average molecular weights in the range of about 15,000 to about 25,000. Although cured neat films of these compositions are tough, the partially cured or B-stage product tends to be brittle. There accordingly remains a need in the art for improved, less brittle thermosetting adhesives for use with thermoplastic compositions.